1. Field of the Invention
The present invention relates to aircraft having adjustable wings, and, more particularly, to aircraft having forward-swept wings which are rotatably mounted to the fuselage to allow variation in the angle of incidence of the wings.
2. Prior Art
A forward-swept wing configuration provides desirable flying and control characteristics at low aircraft speeds. Therefore, it is especially desirable to utilize a forward-swept wing configuration with aircraft designed for short take-off and landing (STOL) use. However, a limiting characteristic of this and other types of aircraft is that the achievable angle of incidence or angle of attack at touchdown is somewhat limited. With fixed wing aircraft, the angle of incidence of the wing is controlled by the orientation of the aircraft fuselage. Thus, the maximum angle of incidence of a fixed wing at touchdown cannot exceed its incidence when the fuselage orientation is such that the tail cone scrapes the ground.
This limitation can be overcome through the use of wings made adjustable with respect to the fuselage so that their angle of incidence can be varied. By increasing the wing incidence with respect to the fuselage by the use of an adjustable wing, the angle of incidence of the wings during landing can be increased past the maximum attainable angle of attack for a fixed wing aircraft. For example, if the maximum allowable fuselage incidence at touchdown is approximately 15.degree., an adjustable wing capable of increasing the incidence of the wing with respect to the fuselage 8.degree.-10.degree. can provide a total angle of attack of approximately 23.degree.-25.degree., thereby increasing the lift achievable by appoximately 50%. By increasing the achievable lift, a reduced landing speed can be utilized without sacrificing maneuverability or increasing the likelihood of stalling.
A few attempts have been made to provide an aircraft having foward-swept wings which are adjustable to vary the angle of incidence. For example, U.S. Pat. No. 2,406,588 discloses an aircraft having forward-swept wings, each wing being divided into a fixed inner wing panel and an adjustable outer wing panel. The wings are divided along a chord which is intersected at its quarter chord point by a line containing the center of gravity of the airplane and perpendicular to the longitudinal axis of the fuselage. The axes of rotation of the outer wing panels are within a common line, but are not contained within a line common with the line containing the center of pressure of the airplane.
A disadvantage of this type of aircraft lies in the fact that a relatively powerful device must be carried by the aircraft to rotate the outer wing panels and overcome the aerodynamic forces counteracting their rotation. In addition, the connecting members about which the outer wing panels rotate must be sufficiently strong to bear the bending moments imparted on the inner wing panels and fuselage by the aerodynamic forces acting upon the outer wing panels, as well as the vertical shear forces. This requires that sufficient structure be utilized to support the axle within the inner wing panels and fuselage. All of the aforementioned structure adds considerable weight to the aircraft, thereby reducing its maximum payload and other performance characteristics.
Another example of a variable incidence winged aircraft is shown in U.S. Pat. No. 2,293,644. This patent discloses a "pusher-type" aircraft having wings which can be rotated forwardly into a forward-swept configuration. In one embodiment of the disclosure, the wings include wing tips which are secured to the ends of their respective fixed wing portion by an inclined pivot. These pivots are inclined in such a manner that, when the movable tip portions are in their rearward position, the rearward portion of each tip portion is slightly above the forward edge thereof, thus giving the movable tip portions a negative angle of incidence. As the tip portions are moved toward the forward position, so that the wings assume a forward-swept configuration, this negative incidence decreases until the incidence of the tip portion is substantially neutral or slightly positive. Thus, in flight, both tip portions would normally be in the rearward position and, during landing, the tip portions would be rotated to their forward positions.
A disadvantage of this wing construction is that wing tip positioning structure is mounted exteriorly of the wings adjacent a leading edge. The presence of this exterior structure approximate the leading edge of the wing increases the wind resistance and drag associated with the wings and also creates undesirable turbulence in the air flowing over the wings which tends to increase the stall speed of the aircraft. In addition, the disclosed aircraft requires extensive mounting structure to provide a sufficient junction between the wings and the fuselage, and auxiliary equipment is required to facilitate the positioning of the wing tips. This increases the overall weight of the plane, as well as the weight of the wings.
Accordingly, there is a need for an aircraft having a forward-swept wing design which is rotatably mounted to the fuselage to permit a variation in the angle of incidence or angle of attack of the wings and which includes a positioning mechanism that is relatively lightweight and uncomplicated.